The present invention relates in general to ferrite shields for suppressing high frequency noise to and from cables and, in particular, to a new and useful ferrite suppressor case which includes internal locking tabs which permanently lock the case closed and which are inaccessible from outside the case.
Ferrite suppressors are manufactured in geometries which use about one cubic inch of ferrous oxide material cast into various cylindrical or rectangular shapes. A hole is provided in the suppressor through which a cable or wire can pass. The cables which use this type of product are data transmission cables carrying data at low processing frequencies from computer sources. Such a wire or cable can act as an antenna by either receiving or transmitting other unwanted frequencies.
Certain Federal Communications Commission (FCC) regulations require suppression or elimination of these unwanted frequencies. Also many computer devices require the same type of suppression to enhance overall system performance. Ferrite shields installed on the cable suppress the higher, unwanted frequency signals while permitting the lower data frequencies to pass unaltered. Thus, the undesirable "antenna characteristic" of a cable is controlled.
An advancement to the original solid ferrite design has been to split it in half, or bisect it. This allows the two halves to be joined over the wire. A coarse and unsophisticated method of holding the halves together has been to simply tape or wrap them with a plastic tie wrap.
U.S. Pat. Nos. 3,223,776 to Piasecki and 3,278,674 to Matthysee et al. show junction enclosing cases having slot shaped openings with a plurality of projections or comb like tines closely engaging multiple conductors extending through the case, regardless of the diameter of the conductors. The clam-shell halves of these cases are locked closed by external tabs which are easily accessible so that the cases can be reopened. These patents do not use ferrite cores for engagement around a conductor.
U.S. Pat. No. 4,825,185 to Matsui shows a split ferrite core that is held in two halves of a case for snapping closed around a conductor to achieve a shielding function with external latch means which may be reopened.
The use of external locking tabs on a clam-shell case for a ferrite core is also disclosed in European patent application 0 257 179 to Nakano.
All of the known case designs for ferrite cores, are reopenable without damaging the case.
Some of these constructions require the use of tools but all provide exterior locks which are accessible from outside the case. Certain situations however, require that the ferrite core not be removable at all. Current German VDE regulations appear to require a case that is non-removable, short of cutting the case or otherwise opening the case only by destroying it. In view of this type of regulation, the only currently available engineering and manufacturing choices are either to seal the external locks with glue or by ultrasonic welding where the plastic of the case is melted; to use an over-molding process where the ferrite core must be preassembled to the circuit before the end terminals are attached, and then to mold a layer of protective covering over the assembled ferrite; or to use a filtered connector at the ends of the circuit.
All of these methods are expensive and require extra components and extra manufacturing operations.